1. Technical Field
The present invention relates to a method of fabricating a semi-insulating GaAs single crystal material with homogeneous impurity concentration such as chromium (Cr) and silicon (Si), produced by a horizontal zone melt method.
2. Background Art
A semi-insulating GaAs single crystal is typically fabricated by a boat method which utilizes a horizontal Bridgman method (HB method) and a gradient freeze method (GF method). These prior art methods, however, suffer from the drawback that they employ so-called "normal freezing" where the entire GaAs in a boat except for a seed crystal is melted, seeded, and then crystallized so that Cr with a small coefficient of segregation is extremely changed in its concentration in the longitudinal direction of the crystal (e.g., the concentration is changed from 10 to 100 times an ordinary value for its rate of solidification up to 0.9). In the case of Si, its concentration is changed to about five times.
A horizontal zone melt method is known to solve such a change of impurity concentration due to the segregation, as disclosed for example in Japanese Patent Application Second Publication No. 14382/1974. The zone melt method is generally adapted to provide a melt zone having a width less than the length of the boat or crystal, and the melt zone is moved while keeping the zone width unchanged from the rear end of the crystal in a direction of the seed crystal, seeds the crystal, and further moves the same from the seeded portion in the direction of the rear end for fabrication of a single crystal. The method makes it possible for impurities such as Cr which produce ordinary segregation to keep, assuming the melt width x and the crystal length 1, the impurity concentration from the seeded portion to a portion of 1-x unchanged.
However, residual Si, which enters the melt zone from the silica boat, is not accompanied by an ordinary segregation phenomenon. That is, the concentration of the residual Si is determined by the oxygen contained in a melt as shown by the following chemical reaction: EQU SiO.sub.2 (S)=Si(in GaAs melt)+20(in GaAs melt) --- (1) EQU K=[Si[0].sup.2
In other words, the higher the oxygen concentration in the GaAs melt, the lower the Si concentration, while the lower the oxygen concentration, the higher the Si concentration. Thus, the HB method and GB method permit the oxygen to be more concentrated at the rear end of the crystal whereby the Si concentration is more reduced at the rear of the same although the segregation coefficient is about 0.14.
In order to keep the electrical characteristics of the Cr-doped semi-insulating crystal unchanged, it is necessary to keep unchanged not only the Cr concentration but also the residual Si concentration. In the aforementioned zone melt method, however, the melt width is narrow. This allows the single crystal portion to incorporate therein oxygen owing to the segregation as the single crystal is grown from the seed crystal portion, even if the oxygen has been dissolved in the melt. Additionally, no further supply of oxygen into the melt can be expected. Thus, the rear portion of the crystal has a reduced oxygen concentration but an increased Si concentration. That is, the conventional horizontal zone melt method suffers from producing a crystal with the residual Si concentration made non-homogeneous lengthwise, although the crystal has the homogeneous Cr concentration.
U.S. Pat. No. 3,623,905 teaches a method of producing a semi-insulating GaAs single crystal. In this prior art method, like the above-mentioned Japanese Patent Application Second Publication (14382/1979), oxygen is intentionally doped to reduce the remaining Si. Therefore, oxygen concentration is high in the melt zone at the rear end of the growth. This raises the problem of sticking. Specifically, the silica boat and the rear end of the crystal stick to each other since oxygen is excessively enriched thereat. If sticking occurs, the silica boat has to be broken to take out the crystal. The silica boat is very expensive, and generally it is expected to be used three to five times.